Witnesses in Nigeria say at least 21 people have been killed in the northeastern city of Maiduguri in the past two days, as government forces press their offensive against Boko Haram militants.

The government says at least eight militants were killed in the embattled city on Thursday. The Friday killings were described as an apparent retaliation against local pro-government vigilantes who had helped government troops identify militants.

Western and Nigerian news reports say militants smuggled weapons used in the Friday attacks past security checkpoints by hiding them in a coffin.

It has not been possible to independently verify either government or rebel claims because of downed telephone networks and restricted access to combat zones in the north.

The military says it has captured more than 150 Boko Haram militants since launching its offensive last month.

The militant group launched its insurgency in 2009, seeking to impose Islamic law in northern Nigeria. Human Rights Watch says more than 3,000 people have been killed since then in Boko Haram-related violence, including hundreds of killings by military forces trying to crush the uprising.